Because who doesn’t like a sweet reward for not ruining the planet?

The next time you remember to bring a reusable bag to the grocery store, you might also be more likely to buy yourself a candy bar. A study of thousands of grocery shoppers found that those who brought their own bag tended to reward themselves with junk food–at the same time that they were more likely to buy organic produce.

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The research, from Uma Karmarkar, an assistant professor at Harvard Business School, and Bryan Bollinger, an assistant professor at Duke University’s Fuqua School of Business, isn’t the first to find that people are more likely to treat themselves after they think they’ve done something virtuous–the so-called licensing effect. But while previous studies have been limited to the lab, this one was able to show the effect in real life.

“One of our reasons for looking at grocery stores is because it’s kind of interesting that you have the opportunity to make many, many different kinds of choices,” says Karmarkar. “It’s this very complicated, habitual situation where so many messages are being sent to you. “Bringing a bag adds in something that’s new for many people. So it could send signals that are strong enough to rise above those messages and have an important impact.”

The researchers combed through data from over 2 million transactions at a grocery store in California, before the state passed a plastic bag ban. Thanks to loyalty cards, the researchers were able to see not only differences between customers, but also how the same customers made different choices on separate trips to the store. Remembering a reusable bag, it turned out, made a clear difference.

When shoppers in the study brought their own bag, they were more likely to choose organic products–from milk to yogurt to kale–over non-organic, probably because the bag makes people think about being “green” or “good,” and buying organic food is a consistent choice to make. Through the separate psychological effect of licensing, they were also more likely to buy treats like ice cream, cookies, or potato chips.

Since the data came from before the state banned plastic bags and began requiring stores to charge 10 cents for paper bags, it’s possible that the effect would be different now. Karmarkar compares it to bottle recycling, which was uncommon until states started adopting bottle bills in the 1970s.

“To put it in a cute phrase, you no longer give yourself a cookie for recycling a bottle,” she says. “It’s become the norm. When bringing bags becomes the norm, it may be that you don’t get a cookie for it.”

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Plastic bag bans are quickly spreading–28 states have or are considering bag bans or taxes in at least some cities–so bringing your own bag may quickly be the norm in most places.

But the study raises a question: If you’re trying to adopt a new, more environmentally responsible behavior, is there a way to make sure that you don’t inadvertently start eating more ice cream at the same time?

“It’s not clear whether people are doing this consciously or unconsciously,” says Karmarker. “One possibility is that people could choose to make it conscious…and indulge yourself in ways that fit other goals. You could say if I bike to work, I’ll get five extra minutes of sleep tonight.”

For grocery stores, the study has another implication: Until bag bans are widespread, maybe stores should be stocking organic kale next to the candy bars in the checkout lane, where people can see it as they’re pulling out their bags.

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About the author

Adele Peters is a staff writer at Fast Company who focuses on solutions to some of the world's largest problems, from climate change to homelessness. Previously, she worked with GOOD, BioLite, and the Sustainable Products and Solutions program at UC Berkeley.